'Easy' to tell if US warship is nuclear - King

When a US warship visits New Zealand later this year, for once Labour leader Annette King will have "full confidence" John Key won't be lying.

Under the law, foreign vessels can only visit New Zealand waters if the Prime Minister is satisfied they're not nuclear-powered or carrying nuclear weapons.

The United States' policy is to neither confirm nor deny either of these things, but Ms King says it won't be hard for officials to figure it out.

"It's up to the Prime Minister of course to guarantee that visiting warships from any country, not just the United States, are not nuclear-powered or carrying nuclear weapons, and I think it'll be relatively easy for the Prime Minister to do that," Ms King told Newshub.

While the historic visit will take place under a National Government, Ms King says it's a vindication of the Labour Government's call in the 1980s to keep New Zealand nuclear-free.

Former National leader Don Brash infamously told US officials in 2004 that New Zealand's nuclear-free policy would be "gone by lunchtime" if they won the 2005 election.

"There has been doubt about National's commitment to nuclear-free New Zealand as recently as 2006, but it is something New Zealanders feel very strongly about," says Ms King.

"It has been New Zealand that has led in this area for some time, and we can be proud of our long record and history in this area."

The visit will take place in November, another step towards the thawing military relationship between the US and New Zealand. New Zealand's nuclear ban saw it suspended from the ANZUS security agreement.

The US lifted its ban on New Zealand warships in 2012 after both countries took part in military exercises in Hawaii.

Ms King says the thaw began much further back in the wake of the 9/11 attacks and the US invasion of Iraq (which New Zealand did not take part in).

"I think you've seen closer ties develop even under Helen Clark," she says. "The thaw had already begun, and it begun with the assistance for the reconstruction in Iraq and work in Afghanistan, and I think you've seen that continue under this Government."

The visit was formally announced by Mr Key and US Vice President Joe Biden on Thursday.